Overlap image texture on a material
You're currently telling Blender that there's transparency behind your image, but after that it is mixed with a Mix Shader that is a 50/50% mix between your wall background and your image. In order to put your image over the wall you need to plug the Alpha output of your image into the Factor socket of the Mix Shader, not into the Alpha socket of the Principled BSDF, so basically, if I simplify, it should look like that: